Seat Foam Calculator
Turn cushion dimensions into a foam volume in cubic metres and board-feet, and see the finished mass at the density you pick.
Foam block
The block is a rectangular approximation. Real cushions taper and contour, so order a little extra thickness for shaping and compression under the trim cover.
Volume, density, and how a seat lasts
When you rebuild a seat, two numbers do most of the work: how much foam the cushion takes, and how dense that foam is. Volume is simple geometry — length by width by thickness — and it tells you how big a block to order, whether you buy by the cubic metre or by the board-foot.
Density is what separates a cushion that holds up from one that flattens in a season. Because density is a mass per unit volume, multiplying it by the block volume gives the finished mass, a handy cross-check against a foam’s spec and a fair way to compare grades. This tool reports both so you can size the block and weigh up the trade-off between cost and longevity.
Frequently Asked Questions
How is foam volume calculated?
The cushion is treated as a rectangular block: length × width × thickness. Entered in centimetres, that gives a volume in cubic metres, which the tool also converts to board-feet — the unit upholstery foam is often sold and priced in (one board-foot is a 12 × 12 × 1 inch piece).
What does foam density mean, and why does it matter?
Density is the mass of foam per cubic metre (kg/m³), and it's the single best guide to how a foam behaves. Higher-density foam supports more weight before bottoming out and keeps its shape far longer, which is why it costs more. Density is separate from firmness — a dense foam can still be made soft — but for a seat you'll sit in daily, density is what predicts durability.
How is the mass figure useful?
Multiplying volume by density gives the finished mass of the block. It's a quick way to sanity-check a foam grade against its spec sheet and to compare two options: if a cushion of the same size comes out much lighter, the foam is lower-density and will likely soften sooner.
What density should I choose for a car seat?
Automotive and furniture seat foams commonly fall around 30–55 kg/m³, with the higher end used where a cushion takes constant load. Always check the specific foam's data — this tool doesn't recommend a grade, it just shows what volume and mass your chosen density implies.
Why order a bit more thickness than the block suggests?
A finished cushion tapers, contours, and compresses under its trim cover, and foam is trimmed and glued during shaping. Real cushions rarely match a plain rectangle, so treat the block as a minimum and buy a little extra thickness for sculpting.
Estimates for planning — not professional automotive or installation advice. Check the specific foam’s data sheet for its density and load rating.